Go to the location of the first Intel Piece, located at the top of the ladder in the first church. Get the Intel Piece, and fall back down. You will die and start out getting the Intel Piece again. The Intel Piece keeps adding to the total number that you have. Repeat this process until you have 30 Intel Pieces.

Easy experience

To level up faster than usual, try to use the same gun until all the marksman and elite challenges are completed. They provide huge boosts and will quickly level you up to get new guns. However, do not use those guns yet; just keep on the challenge until it has completed. The third phase of the challenge usually ends with 1,000 experience points, which usually increases you a rank in itself. Also, do not forget about the strange challenges like jumping 15 feet and surviving or falling 30 feet to your death. To complete the fall 30 feet to your death challenge if not previously unlocked, play the "Countdown" map, and jump into one of the open missile silos. You will get instant experience points for simply dying. Also, remember to go prone and crouch to get those challenges as well.

To level up, gain points, and complete challenges easily, find a TDM game on the "Shipment" map, preferably with a lot of people. Make sure you have the Steady Aim/Martyrdom/Stopping Power Perks enabled, and choose a gun you want to level up. Shipment is a very small close shoulder-to-shoulder map. Just run around holding down the trigger, taking down as many people as possible. This is very fast paced, and you should get points very quickly.

Play games such as Sabotage and Ground War to get lots of experience points because both games offer many kills. Sabotage has a long clock, giving you the opportunity to get many kills or challenges. Also, look for Sabotage on the "Wetworks" map. That map takes a lot of time up.

Better accuracy

Once you get the Red Dot Scope add-on to your weapons, use it over the ACOG scope. The red dot will not enhance or take away anything and provides a better aim point on most weapons. The ACOG scope will actually add more range to your weapon, but take away accuracy which accomplishes almost nothing, especially for snipers because it takes away range and adds nothing.

Better sniping

If a target is moving, make sure to lead it. If you are not making shots, try to lead your target more. Also, if you are a skilled sniper, try using the M40A3 over the M21. It has a slower reload time and rate of fire, but it has enough damage to kill a target in one shot instead of two or three. It also allows you to be less noticeable with less rounds being fired so someone else cannot follow the tracer shot back to you. A good setup for the sniper is the Claymore Perk to alert you when an enemy is trying to close in on your rear, and either the Sleight Of Hand or Stopping Power, then Iron Lungs to give you more time to line up your shots.

"No Fighting In The War Room" mission checkpoints

Reach the listed location in the "No Fighting In The War Room" mission with at least the indicated amount of time left to get a checkpoint:

Checkpoint 1: The corner after the dining area/kitchen with 8 minutes 15 seconds
Checkpoint 2: Half way down the stairs with 7 minutes 45 seconds
Checkpoint 3: After the large area of creates and before the small corridor with 6 minutes 15 seconds
Checkpoint 4: Before the slow opening door with 3 minutes 30 seconds
Checkpoint 5: Just before you blow up the wall to the war room with 1 minute 15 seconds

Extra ammunition for R700 sniper in Multiplayer mode

At level 38 in Multiplayer mode, you get the Overkill Perk Class 2 (located in the Perk 2 or Red Perks). This allows you to carry two primary weapons rather than a pistol. Some of the guns in online play can share ammunition. If you are a skilled sniper and prefer the bolt action to a semi-auto sniper, you can share ammo between the R700 (unlocked at level 34) and the G3 assault rifle (unlocked at level 25). Instead of having 40 rounds, you will have 152 rounds for the sniper, effectively giving you an endless supply of ammo for the R700. If you are a G3 user, you will gain an extra 40 rounds (two clips). This works both ways; since you share ammunition, you might not want to use too much ammo with the G3. Also, there are more combinations than just this one; experiment with different guns to see which ones can share ammo.

When you reach level 46, you will get the M14 Assault Rifle. In the class that you want to have the extra ammunition, you will need the M14 Assault Rifle and M21 Sniper Rifle as weapons. To use those two weapons, you must have Overkill (obtained at level 38) and your second Perk on Overkill. For the other two Perks, you can have whatever desired.

Good sniping location in Crossfire map in Multiplayer mode

This spot can only be reached when you are playing the old school free-for-all section. Go to the building at the very end of the map on the opposite side of the bus. You can get to the very top of the building that is around the corner. First, get to the second story where the chair is found. Get on the top of the chair facing the wall, and jump. You may have to run and jump, but eventually you will catch an edge. You can then work your way up to a great sniping position.

Dog attack in "All Ghillied Up" mission

In the "All Ghillied Up" mission, you will come across a wild dog eating a dead soldier's remains. You will be told to leave it alone, but if you want a very difficult challenge in the game, shoot it. You will be attacked by a large pack of angry dogs.